(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device where positioning of a flexible printed circuit connected to the liquid crystal display panel is improved.
(2) Description of the Related Art
Liquid crystal display modules (liquid crystal display devices) are used as the display device of various portable information apparatuses, such as portable phones, portable information terminals (PDA's), digital cameras and multimedia players.
The demand is high for reducing the thickness and narrowing the frame of these liquid crystal display modules, such as portable phones. Therefore, as shown in JP2010-276808A, in the backlight portion for supplying backlight to the liquid crystal display panel in the liquid crystal display module, no space for incorporating light emitting diodes, which are the light source of the backlight, into the backlight portion can be secured, and thus a structure where the light emitting diodes are mounted on the flexible printed circuit, which is externally connected, is adopted.
FIG. 1 shows a liquid crystal display panel that is formed of a liquid crystal layer sealed between two glass substrates (SUB1, SUB2) and provided within a mold frame MFR made of a resin. In addition, a flexible printed circuit (FPC) for a liquid crystal display panel that supplies an electrical signal to the liquid crystal display panel and the semiconductor chip (DC) for driving the display pixels is connected to and provided on the liquid crystal display panel. In the flexible printed circuit, an interface terminal portion 1 is formed for the liquid crystal display panel in such a manner as to extend straight from the liquid crystal display panel, part of the flexible printed circuit is folded to cover the rear surface, and the portion 2 on which light emitting diodes are mounted is secured to a predetermined location within the mold frame MFR.
FIG. 3 is a cross sectional diagram along single-dotted chain line A-A′ in FIG. 1. The liquid crystal display panel is secured to the mold frame MFR with a double-sided adhesive tape 4, the light emitting diodes LED are mounted on the flexible printed circuit FPC, and the flexible printed circuit FPC is secured to the mold frame using the double-sided adhesive tape 4. In the liquid crystal display panel, an upper polarizing plate UPO and a lower polarizing plate SPO are pasted to the outside of the glass substrates. In addition, a reflective sheet RS, a light guiding plate LG and an optical sheet OS formed of a prism sheet and a diffusion sheet, which form a backlight, are provided in a predetermined location within the mold frame.
As shown in FIG. 2, the part of the flexible printed circuit on which light emitting diodes LED are mounted is positioned by boss pins 3 provided on the mold frame MFR or the light guiding plate LG. Through holes are created in the locations of the flexible printed circuit FPC that correspond to the boss pins 3.
FIGS. 4 and 6 are diagrams showing other embodiments where light emitting diodes LED are mounted on a flexible printed circuit for a liquid crystal display panel. In FIG. 4, the entirety of the flexible printed circuit is folded towards the backlight (rear side of the mold frame), and an interface terminal portion 10 for a liquid crystal display panel is formed so as to extend from the rear of the mold frame MFR to the side.
FIG. 6 is a cross sectional diagram along single-dotted chain line B-B′ in FIG. 4. The portion 2 of the flexible printed circuit FPC on which light emitting diodes LED are mounted is secured to the mold frame MFR in a predetermined location on the rear side. The portion 2 is positioned by boss pins 3 formed on the mold frame MFR or the light guiding plate LG.
FIGS. 7 and 8 show how the portion of a conventional liquid crystal display module on which light emitting diodes are mounted is assembled. A double-sided adhesive tape 4 is pasted to the rear surface of the mold frame MFR in a predetermined location. Next, the portion 2 of the flexible printed circuit FPC on which light emitting diodes are mounted is secured to the mold frame by positioning the through holes 30 in the flexible printed circuit FPC to the boss pins 3 formed on the mold frame. Thus, the boss pins 3 and the through holes 30 make the assembling process easy, and it is also possible to precisely position the portion on which light emitting diodes are mounted. FIG. 8 is a perspective diagram showing the portion on which light emitting diodes are mounted in FIG. 2 and how it looks after the pasting process in FIG. 7.
FIG. 9 is a side diagram showing a boss pin 3 and the portion around it, which shows that the boss pins 3 for positioning protrude the most on the rear surface of the liquid crystal display module in such a state that the portion 2 on which light emitting diodes are mounted is pasted to the mold frame MFR with the double-sided adhesive tape in between. In FIG. 9, which illustrates how the boss pin protrudes, h is the length by which the boss pin protrudes from the surface of the flexible printed circuit and d is the diameter of the boss pin. In general, h≧0.1 mm and d≧0.5 mm.
Though it is necessary to make the length h of the boss pin 3 as short as possible, particularly in order to make the liquid crystal display device thinner, the process for positioning the portion 2 becomes inefficient when h is short. In addition, when stress is applied to a boss pin 3 during or after the incorporation of the liquid crystal display device into the main body, such as of a portable phone, stress is applied to the liquid crystal display panel held in the mold frame MFR as well, and there is a risk of a glass substrate that forms the liquid crystal display panel being damaged.